The present invention relates to an incoming call transfer terminal connected for use with communication networks such as an ISDN (Integrated Services Digital Network) operating according to the call control method defined by the I Series Recommendations of the CCITT International Telegraph and Telephone Consultative Committee.
The ISDN is known to be capable of communicating the presence of a call from a specific communication terminal to a called communication terminal using a calling sub-address, i.e., one of the usable elements of information defined by the I Series Recommendations of the CCITT. The calling sub-address is also used to communicate personal identification information by which to specify a person (i.e., name of the calling party). This scheme eliminates one disadvantage of prior art analog telephone networks. That is, old networks let the called party know which terminal is calling and who is calling only after a communication channel had been established between the parties. The ISDN allows such information to be communicated to the called party before the communication channel is set up.
With the ISDN, one communication channel is constituted by two B channels (information channels on which to transmit call-related information at a transmission speed of, say, 64 kb/s) and one D channel (signal channel on which to transmit call control information at a transmission speed of, say, 16 kb/s). Because the capacity of the D channel is sufficiently large, the call control information that may be transmitted includes not only the minimum required control information such as the other party's telephone number, but also a calling number (i.e., the caller's subscriber number) and a predetermined length of other desired information (user-user information). The call control information is transmitted before the called party responds.
The telephone set connected to the ISDN may be equipped with a call-selecting function. That is, the telephone set stores in its built-in memory the subscriber numbers of the parties that the owner of the set is willing to respond to. When a call comes in, a check is made to see if the calling number transmitted over the D channel coincides with any of the numbers stored. A response is allowed only if one of the stored subscriber numbers is found to match the calling number. This keeps off any unwanted sales pitch by telephone and wrong number calls, among others.
With the ISDN, a communication channel is established between a calling and a called terminal based on two kinds of information. One kind of information is the information necessary for setting up a communication channel. It comprises a calling number (i.e., the calling party's subscriber number transmitted initially from the calling terminal to the called terminal), a called number (the called party's subscriber number), the information transmission capability of the network in question, the type of communicating terminals, and channels to be used. The other kind of information is a call-setting message formed from various elements of information for setting the information specific to the calling party such as the calling sub-address and called address.
There are cases in which an incoming call is desired to be transferred to a third party using a terminal capable of such transfer (called the incoming call transfer terminal). In such cases, it is naturally preferable that the call-setting message from the calling terminal would be transferred unchanged to the third party's terminal. In practice, only the call-setting message prepared by the incoming call transfer terminal is transmitted to the third party. There is at present no way of conveying the calling party's call-setting message to the transfer destination.
This is a major disadvantage of the prior art incoming call transfer terminal: its inability to transfer the calling party's call-setting message to a third party when transferring the incoming call to the latter.
On a communication network such as the ISDN capable of transmitting a calling party's subscriber number (calling-number) to a party being called, the called party knows the calling number before responding. This allows the called party to choose from a number of ways to deal with the incoming call. There may be a case in which the called party wants to transfer the incoming call to a third party using another subscriber line via the incoming call transfer terminal. In such a case, however, the calling party's subscriber number is not transferred to the third party. This disadvantage is bypassed conventionally in a number of ways. One way is for the incoming call transfer terminal (practiced typically as an incoming call transfer telephone set) to determine whether or not to transfer the call depending on the calling number received. Another way is for the incoming call transfer telephone set to call the third party using a special ringing tone indicating that the current call is being transferred. Yet another way is for the incoming call transfer telephone set to give the third party a voice message saying that the call is being transferred after the third party has responded and a communication channel is established.
As described, all that the prior art incoming call transfer terminal can do in informing the third party of the call being transferred is to use a special ringing tone or employ a voice message after the third party has responded and the communication channel is established. With the prior art, it is impossible to notify the third party of the calling party's subscriber number (calling number).